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DEMO

The large scale fusion experiment ITER (Latin for "the way") ) is currently under construction in Cadarache in the south of France. ITER is the world’s first fusion reactor, and its aim is to achieve a positive energy balance in the fusion of deuterium and tritium. However, the length of the plasma pulses produced will only be between a few minutes and one hour. In addition, the expected fusion power produced with a power gain of Q = 10 is still too low to be used for net electricity generation. The ITER experiment will nevertheless develop and demonstrate solutions on a practical scale to all fundamental physical and technical issues on the path towards fusion energy.

Net electricity generation will then be the task of a subsequent device – the demonstration reactor DEMO. In comparison to ITER, DEMO will be somewhat larger, have higher fusion power and power gain, and thus be capable of feeding several hundred million watts of electrical energy into the grid. Another of DEMO’s important goals is to achieve higher availability than ITER, which necessitates longer plasma pulses on the one hand and a longer lifetime for all components on the other.

DEMO’s high availability and substantial fusion power will result in considerably higher tritium consumption in comparison to ITER. This super-heavy hydrogen is an unstable isotope and has until now been primarily produced on Earth as a by-product in nuclear fission reactors. The amount of tritium thus available – currently some 10 kg – is sufficient for experimental operations at ITER, but not for DEMO. For this reason, the total amount of tritium required will be produced on site at a DEMO reactor in a “breeding blanket” surrounding the hot fusion plasma. The neutrons emitted from the plasma penetrate the blanket where they react with lithium, producing tritium and helium as a result. The tritium gas formed is then captured and fed into the hot fusion plasma. The second component required for the fusion fuel, deuterium, occurs on Earth in practically inexhaustible quantities in seawater.

According to current knowledge, most concepts for DEMO build on torus-shaped magnetic confinement based on the tokamak principle – as does ITER. In a tokamak, the hot plasma is confined by a magnetic field, which is produced by superconducting coils and a current flowing through the plasma. The major challenge is to maintain stable control of the hot plasma – at temperatures of more than 100 million degrees – and of the heat and particle fluxes escaping from the plasma so that the walls of the vacuum vessel can withstand the interaction with the plasma on a long-term basis.

According to recent European studies, a DEMO tokamak reactor could achieve the following operating parameters:

Plasma volume

2500 m3

Magnetic field strength

5.7 tesla

Plasma current

20 megaampere

Fusion power

2000 MW

Elektrical output power

500 MW

Pulse length

2 hours

Start of operation

approx. 2050

Copyright: KIT Karlsruhe

DEMOdesign study

The development of DEMO will draw on many of the insights gained in the construction and operation of ITER in terms of physics and technology. Nonetheless, work on developing a concept for DEMO must begin today, parallel to work at ITER, so that research activities specific to DEMO can clarify the open questions that ITER is not expected to answer. For instance, the demands placed on the lifetime of materials and the stability of plasma operation will be considerably higher at DEMO than at ITER. The practical realization of a breeding blanket with full tritium self-sufficiency (tritium breeding rate TBR > 1) is crucial to the feasibility of commercial fusion energy. Finally, the requirements on the reliability of plasma control are by far stronger than on ITER, and they will have to be fulfilled using a set of diagnostics and actuators which are quite limited as compared to ITER.

A new European research roadmap for the development of fusion energy defining the key topics for developing DEMO was published in 2012, declaring the goal of realizing commercial energy generation through fusion by 2050 by means of coordinated research activities. In Germany, a close cooperation was initiated in 2010 between the fusion laboratories in Garching, Greifswald, Karlsruhe, and Jülich in order to jointly discuss the state of the knowledge in fusion research and set the course for further research activities with regard to DEMO. Jülich is participating in the coordinated European and German research activities on fusion in the areas of its core competences: plasma-wall interaction, fusion-related materials research, and plasma diagnostics and control. Within the European fusion research, Jülich provides the project leaders for the topics “Materials research” (G. Pintsuk) and “DEMO Diagnostic and Control” (W. Biel).

In Germany, intensive research continues into an alternative concept for confining a hot fusion plasma. In a device known as a stellarator, the magnetic field confining the plasma is produced entirely by the magnetic field coils. This means that, unlike in a tokamak, no plasma current is required, which in principle facilitates plasma discharges of unlimited duration. Jülichis developing measurement technology and preparing and conducting experiments and modelling for the investigation of plasma-wall interaction at the Wendelstein 7-Xstellarator in Greifswald.

Finding solutions to the multifaceted problems associated with DEMO’s development calls for close collaboration between physicists and engineers from various disciplines. Subtasks in this project can also be assigned to students in the form of master’s or PhD theses.